This invention relates to a method of refining a mixed gas used in a rare gas halide excimer laser by sequential treatments of the mixed gas with specially selected reactive and adsorptive agents.
Excimer lasers using rare gas halides represented by ArF, KrF, XeF and XeCl are attracting increasing interest as high-output ultraviolet lasers which will have important applications in the manufacture of semiconductor devices, in photochemical reactions and in many other fields.
Rare gas halide excimer lasers utilize a mixed gas which is comprised of a selected rare gas such as Ar, Kr or Xe, an active halogen source material such as F.sub.2, NF.sub.3 or HCl and an inert diluent gas such as He or Ne. The halogen source material is a highly reactive gas which readily reacts with surrounding materials such as the laser container materials. Therefore, it is inevitable that during operation of a rare gas halide excimer laser the halogen source material in the laser gas is partially consumed in unwanted reactions which give rise to formation of impurity compounds and reduction in the concentration of the halogen source material. In general the impurity compounds formed during operation of the excimer laser are halogen compounds such as CF.sub.4, C.sub.2 F.sub.6, SiF.sub.4, HF, SF.sub.6, CCl.sub.4, CClF.sub.3, CCl.sub.2 F.sub.2 and/or CCl.sub.3 F though the particulars are different depending on the composition of the employed laser gas. As the laser gas is deteriorated in such a manner the output of the excimer laser lowers considerably, so that the excimer laser cannot continuously be operated for a long period of time if no countermeasure is taken.
In industrial applications of any type of rare gas halide excimer laser it is impracticable to simply dispose of the deteriorated laser gas and replace it by a fresh laser gas since very expensive rare gases are used.
Accordingly several methods have been proposed for removal of impurities from an excimer laser gas. The proposals include a condensation method using a cold trap in which the laser gas is cooled until condensation of impurity compounds having relatively high boiling points. However, this method is ineffective for removal of impurity compounds having relatively low boiling points such as CF.sub.4, and this method is not applicable to excimer lasers using a rare gas having a relatively high boiling point such as Kr or Xe because, if applied, the rare gas too undergoes condensation in the cold trap. Also it has been proposed to make the deteriorated laser gas contact with heated metallic calcium to thereby convert the gaseous impurity compounds into solid calcium compounds. However, this method cannot be deemed industrially favorable because of inconveniences of handling metallic calcium and maintaining metallic calcium at a very high temperature such as 650.degree. C. and also because of unreactivity of some impurity compounds with metallic calcium. Another proposal is an adsorption method using either active carbon or a suitable getter material such as a Ti-Zr alloy. However, by this method only limited kinds of impurity compounds can be removed from the deteriorated excimer laser gas while much more kinds of impurity compounds are contained in the same gas.